The present invention relates to nuclear boiling water reactors (BWRs) and more particularly to incorporation of the ATWS (anticipated transients without scram) neutron poison tank into the reactor pressure vessel (RPV).
The stand-by liquid control system is a redundant control system capable of shutting a BWR down from rated power operation to a cold condition in the postulated situation that the control rods cannot be inserted. No operating BWR has required the use of a stand-by liquid control system, though safety considerations dictate that such equipment be available.
The equipment for the stand-by liquid control system is located in the reactor building outside of the drywell and typically consists of a stainless steel storage tank, a pair of full capacity positive displacement pumps and injection valves, a test tank, and the necessary piping, valves, and instrumentation. A detection system of an ATWS also is provided. In terms of signals, the ATWS event is defined as too high a pressure and too high a neutron flux persisting longer than a specified time, or too low of a water level and too high a neutron flux persisting for a defined time.
The stand-by liquid control system is adequate to bring the reactor from a hot operating condition to cold shut-down and to hold the reactor shut-down with an adequate margin when considering temperature, voids, Doppler effect, equilibrium xenon, and shut-down margin. It is assumed that the core is operating at normal xenon level when injection of liquid control chemical is needed.
The liquid control chemical used in boron, typically in the form of sodium pentaborate solution. It can be injected into the bottom of the core where it mixes with the reactor coolant. In present designs, electric heaters automatically keep the solution above the saturation temperature. The system temperature and liquid level in the storage tank are monitored and abnormal conditions are enunciated in the control room.
Making such system passive poses some problems. Present configurations utilize an accumulator at a higher pressure than the postulated maximum reactor pressure. There are a number of disadvantages to this configuration. The neutron poison tank is a high pressure one and associated piping in the containment present costs which are non-negligible. Additionally, space has to be provided in the wetwell to accommodate the nitrogen that is used to pressurize the accumulator tank.